To access subsurface reservoirs of hydrocarbons, water, geothermal heat, or other valuable resources, drillers create boreholes through the intervening rock formations, typically lining them with steel casing and inserting steel production tubing to define one or more flow paths for fluid to enter or leave the borehole. Complex wells with many stages may have multiple casing and tubing strings in a nested, concentric arrangement within the borehole. Herein, the term “pipe” is used to refer generically to all types of borehole tubing including casing, drill strings, production strings, intervention strings, continuous/coiled tubing, and threaded tubing.
As the subsurface reservoirs often contain dissolved salts or acidic fluids at elevated temperatures, the exposed pipe tends to corrode. If such corrosion is not monitored and corrected or otherwise accounted for, it can lead to isolation failures. Even if such failures don't rise to the level of a well blowout (although that is a possibility), they can cause fluid loss and potentially catastrophic formation damage. Thus a number of downhole tools have been developed to inspect for pipe corrosion, including calipers, cameras, acoustic tools, and electromagnetic tools. Of these, only the electromagnetic tools enable monitoring of corrosion of all the pipes in a nested, concentric arrangement. The other tools require the inner pipes to be removed to enable inspection of the outer ones.
Electromagnetic tools normally include multiple transmit and/or multiple receive antennas to acquire measurements with different antenna spacings, and may also (or alternatively) acquire measurements at different frequencies. Because measurements at different frequencies and different antenna spacings correspond to different regions of sensitivity, they can be inverted to determine the wall thickness of each pipe in a nested arrangement. Those pipes having their wall thickness reduced (relative to their initial, nominal thickness) beyond a threshold have likely been weakened by corrosion and may require replacement or other remedial action.
For many, if not all, nested pipe arrangements, the inversion process is computationally intensive and cannot feasibly be conducted in real time as the measurement data is acquired. Yet it is desirable for the logging service providers and their customers to obtain some form of immediate feedback to verify that the necessary measurements are being acquired and to enable prompt corrective action. Further, if the feedback enables the provider to identify potentially problematic regions, the inversion process can be shortened and expedited by focusing the inversion solely on the problematic regions.
It should be understood, however, that the specific embodiments given in the drawings and detailed description thereto do not limit the disclosure. On the contrary, they provide the foundation for one of ordinary skill to discern the alternative forms, equivalents, and modifications that are encompassed together with one or more of the given embodiments in the scope of the appended claims.